Tunable light emission from carbon dots by controlling surface defects

Carbon dots (CDs) are metal-free fluorescent materials that can be used in optical and electronic devices, but few studies have focused on one-step synthesis routes for CDs with tunable color and high photoluminescence quantum yield (PLQY). Herein, CDs with tunable light emission were synthesized using a novel amide-assisted solvothermal approach. The as-prepared CDs were well dispersed and homogeneous, with average diameters of approximately 2.0–4.0 nm, depending on the dopants. Owing to the surface states with different ratios of nitrogen- and oxygen-related species, different CDs can exhibit blue, green, red, or white emission with relatively high PLQYs of 61.6%, 41.3%, 29.1% and 19.7%, respectively. XPS measurements, in conjunction with DFT calculations, indicate that nitrogen substitution (pyridinic/pyrrolic nitrogen) dominates the blue emission, while introducing oxygen functional groups lowered the LUMO energy level, which resulted in redder emission. In addition, the CDs are demonstrated as a bioimaging probe in both in vitro and in vivo assays, and the white light CDs have been demonstrated to be potential fluorescent materials for white-light-emitting diode (WLED).